I am a new Glock owner and have been attempting to learn about the different versions, compatibility, etc. For example: I just bought a gently used Gen4 G17 but it does not have the adjustable back strap. Is there several versions of the Gen4. I have surfed the net bought a book and joined GLOCKTALK. I know there ought to be at least a couple of opinions on this matter here on GT. Any takers? Thanks for the welcome by the way.